Traditionally, there has been a power supplying technology utilizing electromagnetic inductance, and the one utilizing electromagnetic wave. In addition to these, a wireless power-supply technology utilizing a magnetic resonant state has been suggested in recent years.
The power-supply technology utilizing this magnetic resonant state (also known as magnetic resonance, magnetic field resonance, magnetic field resonance) enables transmission of energy (power) by means of coupling magnetic fields of two resonators which resonate with each other. This wireless power-supply technology utilizing a magnetic resonant state enables energy (power) transmission of a longer distance than the wireless power-supply technology utilizing electromagnetic inductance.
For example, PTL 1 discloses a wireless power-supply system which changes a resonance frequency of a power transmission resonance coil and that of a power-reception end resonance coil so as to maintain the resonant state by varying the coupling strength between the power transmission resonance coil and the power-reception end resonance coil, thereby preventing deterioration in the power transmission efficiency from a power transmission device to a power-receiving device, even when the distance between the power transmission resonance coil and the power-reception end resonance coil varies. Further, PTL 2 discloses a wireless power device which varies the coupling strength between a power transmission coil and a power-receiving coil thereby enhancing the power transmission efficiency of the entire device. Further, PTL 3 discloses a power supply system capable of conducting contactless power supply, which includes a power-supplying end resonance coil and a power-receiving end resonance coil between a power-supplying coil and a power-supplying coil, and which is capable of maintain or improve the power supplying efficiency irrespective of variation in the distance between the power-supplying end resonance coil and the power-receiving end resonance coil.